Jason Brooks

100% phosphorescence quantum efficiency of Ir(III) complexes in organic semiconductor films

We demonstrate that three Ir(III) complexes used as principal dopants in organic electrophosphorescent diodes have very high photoluminescence quantum efficiency (ηPL) in a solid-state film. The green emitting complex, fac-tris(2-phenylpyridinato)iridium(III) [Ir(ppy)3], the red-emitting bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonato)iridium(III) [Btp2Ir(acac)], and the blue complex bis[(4,6-difluorophenyl)pyridinato-N,C2](picolinato)iridium(III) (FIrpic) were prepared as codeposited films of varying concentration with 4,4′-bis(N-carbazolyl)-2,2′-biphenyl, a commonly used host material. The maximum ηPL values for Ir(ppy)3, Btp2Ir(acac), and FIrpic were, respectively, 97%±2% (at 1.5mol%), 51%±1% (at 1.4mol%), and 78%±1% (at 15mol%). Furthermore, we also observed that the maximum ηPL of FIrpic reached 99%±1% when doped into the high triplet energy host, m-bis(N-carbazolyl)benzene, at an optimal concentration of 1.2mol%.

Saturated deep blue organic electrophosphorescence using a fluorine-free emitter

We demonstrate saturated, deep blue organic electrophosphorescence using the facial- and meridianal- isomers of the fluorine-free emitter tris(phenyl-methyl-benzimidazolyl)iridium(III)(f-Ir(pmb)3 and m-Ir(pmb)3, respectively) doped into the wide energy gap host, p-bis(triphenylsilyly)benzene (UGH2). The highest energy electrophosphorescent transition occurs at a wavelength of λ=389nm for the fac- isomer and λ=395nm for the mer- isomer. The emission chromaticity is characterized by Commission Internationale de l’Eclairage coordinates of (x=0.17,y=0.06) for both isomers. Peak quantum and power efficiencies of (2.6±0.3)% and (0.5±0.1)lm∕W and (5.8±0.6)% and (1.7±0.2)lm∕W are obtained using f-Ir(pmb)3 andm-Ir(pmb)3 respectively. This work represents a departure from previously explored, fluorinated blue phosphors, and demonstrates an efficient deep blue/near ultraviolet electrophosphorescent device.

Highly efficient blue-emitting cyclometalated platinum(II) complexes by judicious molecular design

Luminescent properties of cyclometalated Ir and Pt complexes have been the focus of considerable research, driven in large part by their potential use as emitters in organic lightemitting diodes (OLEDs). This class of phosphorescent emitters has demonstrated the ability to harvest both electrogenerated singlet and triplet excitons, resulting in a theoretical 100 % electron-to-photon conversion efficiency. Driven by the technological need for full-color displays and solid-state lighting applications, the development of stable and efficient Ir and Pt complexes that emit in the range of 400–460 nm (blue region) is vital. Thus far, the approach to achieve efficient blue-phosphorescent OLEDs has focused on Irbased complexes with either high triplet energy cyclometalated ligands, such as 4,6-difluorophenylpyridine, or electronwithdrawing ancillary ligands, such as picolinate and tetrakis(1-pyrazolyl)borate. There are comparatively few reports on deep blue phosphorescent emitters with fluorine-free cyclometalating ligands, despite potential for improved optoelectronic stability compared to fluorinated derivatives. An example of such a class of materials is metal complexes cyclometalated with the methyl-2-phenylimidazole (pmi) ligand and related analogues that are coordinated to the metal through a neutral carbene. Several Ir complexes have been reported to have efficient deep blue phosphorescent emission at room temperature, including mer-tris(Ndibenzofuranyl-N-methylimidazole) iridium(III) [Ir(dbfmi)], tris(1-cyanophenyl-3-methylimidazolin-2-ylidene-C,C2’) iridium(III) [Ir(cnpmic)], and mer-tris(phenyl-methyl-benzimidazolyl) iridium(III) [m-Ir(pmb)3]. [7c] However, these complexes suffer from either long luminescent decay or relatively low quantum efficiency compared to Ir complexes based on the cyclometalated 2phenylpyridine ligand that have quantum efficiency F of 0.8– 1 and a luminescent lifetime t of 1–5 ms. This difference can be attributed to the combined effects of a high non-radiative decay rate (knr) and low radiative decay rate (kr), which are dictated by the intrinsic properties of the selected metal complex system. Thus, it will be highly desirable to identify rational design motifs that can improve the luminescent properties of deep blue phosphorescent emitters. Compared to Ir analogues, there are relatively few reports on platinum complexes cyclometalated with phenylimidazole carbene ligands. However, one such compound, platinum(II) bis(methylimidazolyl)benzene chloride (Pt-16), has demonstrated impressive device performance with a maximum external quantum efficiency (EQE) of 15.7% and Commission Internationale de L clairage (CIE) coordinates of (0.16, 0.13). Moreover, Pt complexes can provide additional structural variation owing to the square-planar configuration allowing ligands to be designed that are bidentate, tridentate and tetradentate. These variations can significantly alter the ground and excited state properties of Pt complexes. Herein, we report (pmi)Pt-based complexes that demonstrate a higher luminescent quantum yield and faster radiative decay process than published Ir carbene analogues. A new class of Pt complexes with tetradentate ligands have been synthesized. The complexes have a conventional cyclometalated fragment bridged with oxygen to an LL chelating group, where LL is an ancillary chelate, such as, phenoxyl pyridine (POPy) or carbazolyl pyridine (CbPy). The structures of Pt[pmi-O-POPy], Pt[pmi-O-CbPy], and Pt[ppz-OCbPy] are shown in Scheme 1, and are denoted as PtOO7,

High-efficiency yellow double-doped organic light-emitting devices based on phosphor-sensitized fluorescence

We demonstrate high-efficiency yellow organic light-emitting devices (OLEDs) employing [2-methyl-6-[2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2) as a fluorescent lumophore, with a green electrophospho- rescent sensitizer, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] co-doped into a 4,4′-N,N′dicarbazole-biphenyl host. The devices exhibit peak external fluorescent quantum and power efficiencies of 9%±1% (25 cd/A) and 17±2 lm/W at 0.01 mA/cm2, respectively. At 10 mA/cm2, the efficiencies are 4.1%±0.5% (11 cd/A) and 3.1±0.3 lm/W. We show that this exceptionally high performance for a fluorescent dye is due to the ∼100% efficient transfer of both singlet and triplet excited states in the doubly doped host to the fluorescent material using Ir(ppy)3 as a sensitizing agent. These results suggest that 100% internal quantum efficiency fluorescent OLEDs employing this sensitization process are within reach.

Photophysics of Pt-porphyrin electrophosphorescent devices emitting in the near infrared

The triplet annihilation dynamics of near infrared organic light-emitting devices are studied with peak electrophosphorescence at a wavelength of 772nm using a platinum-porphyrin derivative Pt(II)-tetraphenyltetrabenzoporphyrin as dopant. Both the photoluminescent decay transients of the thin films and the quantum efficiency versus current density characteristics of devices using tris(8-hydroxyquinoline) aluminum or 4,4′-bis(N-carbazolyl)biphenyl (CBP) as hosts are fitted by a model based on triplet-triplet annihilation. When the phosphor is codoped with Ir(III) bis(2-phenyl quinolyl-N,C2′) acetylacetonate in CBP, the quantum efficiency is enhanced, and the observed decrease of efficiency at high current densities is explained by field-induced charge pair dissociation. The external quantum efficiency has a maximum of (8.5±0.3)%, decreasing to (5.0±0.3)% at 1mA∕cm2.

Improved initial drop in operational lifetime of blue phosphorescent organic light emitting device fabricated under ultra high vacuum condition

A blue phosphorescent organic light emitting device fabricated under the ultra high vacuum (UHV) condition of 6.5 × 10−7 Pa decreases the initial luminance drop upon lifetesting under a constant dc current of 15 mA/cm2 by 3 times compared to a device fabricated under a high vacuum (HV) condition of 7.6 × 10−6 Pa resulting in a 23% increase in half lifetime. We calculate a water content of 10−4 mol. % in the UHV device emissive layer (EML) and 10−2 mol. % in the HV device EML. We discuss the effects of water on luminance loss and voltage rise for the devices.

Printable phosphorescent organic light-emitting devices

— A new approach to full-color printable phosphorescent organic light-emitting devices (P2OLEDs) is reported. Unlike conventional solution-processed OLEDs that contain conjugated polymers in the emissive layer, the P2OLEDs emissive layer consists of small-molecule materials. A red P2OLED that exhibits a luminous efficiency of 11.6 cd/A and a projected lifetime of 100,000 hours from an initial luminance of 500 cd/m2, a green P2OLED with a luminous efficiency of 34 cd/A and a projected lifetime of 63,000 hours from an initial luminance of 1000 cd/m2, a light-blue P2OLED with a luminous efficiency of 19 cd/A and a projected lifetime 6000 hours from an initial luminance of 500 cd/m2, and a blue P2OLED with a luminous efficiency of 6.2 cd/A and a projected lifetime of 1000 hours from an initial luminance of 500 cd/m2 is presented.

Ink‐jet‐printable phosphorescent organic light‐emitting‐diode devices

Abstract— A novel method for the fabrication of ink-jet-printed organic light-emitting-diode devices is discussed. Unlike previously reported solution-processed OLED devices, the emissive layer of OLED devices reported here does not contain polymeric materials. The emission of the ink-jet-printed P2OLED (IJ-P2OLED) device is demonstrated for the first time. It shows good color and uniform emission although it uses small-molecule solution. Ink-jet-printed green P2OLED devices possess a high luminous efficiency of 22 cd/A at 2000 cd/m2 and is based on phosphorescent emission. The latest solution-processed phosphorescent OLED performance by spin-coating is disclosed. The red P2OLED exhibits a projected LT50 of >53,000 hours with a luminous efficiency of 9 cd/A at 500 cd/m2. The green P2OLED shows a projected LT50 of >52,000 hours with a luminous efficiency of 35 cd/A at 1000 cd/m2. Also discussed is a newly developed sky-blue P2OLED with a projected LT50 of >3000 hour and a luminous efficiency of 18 cd/A at 500 cd/m2.

Comparison of blue-emitting phosphorescent dopants: effect of molecular energy levels on device efficiency

Two blue-shifted iridium phenyl-pyridine dopants are compared in identical device structures. While the dopants have very similar optical behavior, it is found that the device efficiencies are very different and dependent on the host material. Upon comparison of molecular energy levels it is proposed that the electronic properties of the dopant influence the device efficiency through an electron trapping mechanism. It is believed that the relative energetics between the host and dopant play an integral role in the operation of the device.

Modifying Emission Spectral Bandwidth of Phosphorescent Platinum(II) Complexes Through Synthetic Control

The design, synthesis, and characterization of a series of tetradentate cyclometalated Pt(II) complexes are reported. The platinum complexes have the general structure Pt(ppz-O-CbPy-R), where a tetradentate cyclometalating ligand is consisting of ppz (3,5-dimethyl-1-phenyl-pyrazole), CbPy (carbazolylpyridine) components, and an oxygen bridging group. Variations of the R group on the pyridyl ring with various electron withdrawing and donating substituents are shown to have profound effects on the photophysical properties of Pt complexes. Electrochemical analysis indicates that reduction process occurs mainly on the electron-accepting pyridyl group, and the irreversible oxidation process is primarily localized on the metal-phenyl portions. The studies of their photophysical properties indicate that the lowest excited state of the platinum complexes is a ligand-centered 3π-π* state with minor to significant 1MLCT/3MLCT character and are strongly dependent on the nature of the electron-accepting pyridyl moiety. A systematic study of the substituent effects on the pyridyl ring demonstrates that the T1 state properties can be tuned by altering the functionality and positions of substituents. Importantly, it is revealed that how the emission spectra of the Pt(II) complexes can be significantly narrowed and why it can be achieved by incorporating an electron-donating group on the 4-position of the pyridyl ring. Most of the Pt(II) complexes reported here are highly emissive at room temperature in dichloromethane solutions (Φ = 1.1-95%) and in doped PMMA films (Φ = 29-88%) with luminescent lifetimes in the microsecond range (τ = 0.6-13.5 μs in solution and 0.9-11.3 μs in thin film respectively) and λmax = 442-568 nm and 440-544 nm in solution and thin film, respectively. Moreover, these complexes are neutral and thermally stable for sublimation, indicating that they can be useful for display and solid-state lighting applications.